Techniques have been developed for printing or coating paper webs with patterns of additional material. These prior techniques have included printing with gravure presses, blade coating, roller coating, silkscreening and stenciling.
U.S. Pat. No. 4,968,534 to Bogardy describes a stenciling apparatus wherein a continuous stencil comes into intimate contact with a paper web during application of an ink or the like. The apparatus includes an arrangement which draws air through the stencil prior to the application of the ink. The mechanical arrangement is such that to change the pattern, the stencil must be changed. Additionally, such apparatus are unworkable at the wet-end of paper-making machines.
In the related, commonly assigned application, U.S. Ser. No. 07/847,375, an embodiment of a moving orifice applicator is disclosed which includes an elongate "cavity block" or chamber and a perforated endless belt whose lower traverse passes along the bottom portion of the chamber. The chamber is positioned obliquely across a web-forming device (such as a Fourdinier wire). In operation, a slurry of additional material is continuously supplied to the chamber as the endless belt is looped through the bottom portion of the chamber such that plural streams of material are generated from beneath the chamber to impinge the web passing beneath the chamber. As a result, bands of additional material are applied repetitively to the web. The orientation, width, thickness and spacing of the bands are all determinable by the relative speed and orientation of the endless belt to the moving web.
Preferably, the pattern of additional material is applied as uniformly as possible so as to render consistent product across the entire span of the web. However, Fourdrinier machines are very wide (approximately 10 to 20 feet or more) and that circumstance creates the need to extend the slurry chamber to extreme lengths. Accordingly, fluid conditions, particularly pressure, at one end of a slurry chamber may differ significantly from those at the other. Significantly, we have discovered that variations in pressure can cause the fluid discharge from the orifices to vary significantly as the orifices move from one end of the chamber to the other.
It is believed that as the belt progresses through the slurry chamber, its motion imparts a pumping action upon the slurry. Unless corrective measures are undertaken, this action tends to increase fluid pressure at the downstream end of the chamber (where the belt exits the chamber). The motion of the belt may also create a region of low pressure where the belt enters the chamber. Additionally, the very end portions of the chamber itself tend to impart flow disturbances. All these circumstances can create undesireable variations in the discharge of slurry along the slurry chamber and manifest imperfections in the paper product being manufactured.
In U.S. Ser. No. 07/847,375, slurry is introduced into the chamber at a plurality of spaced-apart locations along the chamber. However, the slurry may be introduced such that it, too, creates local fluid disturbances which can be problematic to uniformity.
When using the applicator in constructing banded cigarette papers, the add-on material is usually a form of fibrous cellulose. Such material tends to collect at or about edges and corners of the apparatus within the chamber. If the collections are allowed to accumulate, they can partially or totally clog the perforations of the endless belt and create other problems that disrupt proper and efficient operation of the applicator.
We have also come to realize that unless precautions are undertaken, the belt may entrain bits of the slurry and carry them out of the chamber. Because the belt moves so quickly, this extraneous slurry is soon thrown from the belt, especially where the path of the belt changes direction. Such action creates spots and other blemishes on the final product, exacerbates machine cleaning requirements and may accelerate wear and tear in the applicator.